Many physiological processes require that cells come into close contact with and adhere to other cells or the extracellular matrix. Cell-cell and cell-matrix interactions are mediated through several families of intercellular adhesion molecules or “ICAMs.”
ICAM-1 is a 110 kilodalton member of the immunoglobulin superfamily (Simmons et al., 1988, Nature (London) 331: 624-627) that is expressed on a limited number of cells and at low levels in the absence of stimulation (Dustin et al., 1986 J. Immunol. 137, 245-254). Upon stimulation with inflammatory mediators, a variety of cell types in different tissues express high levels of ICAM-1 on their surface (Springer et. al. supra; Dustin et al., supra; and Rothlein et al., 1988, J. Immunol. 141: 1665-1669). Cells which can express ICAM-1 upon stimulation include non-hematopoietic cells such as vascular endothelial cells, thymic and other epithelial cells, and fibroblasts; and hematopoietic cells such as tissue macrophages, mitogen-stimulated T lymphocyte blasts, and germinal center dendritic cells in tonsils, lymph nodes, and Peyer's patches. ICAM-1 induction occurs via increased transcription of ICAM-1 mRNA (Simmons et al., supra), which is detectable at 4 hours post-induction and peaks at 16-24 hours post-induction.
In vitro studies have shown that antibodies to ICAM-1 block adhesion of leukocytes to cytokine-activated endothelial cells (Boyd et al., 1988, Proc. Natl. Acad. Sci. USA 85: 3095-3099; Dustin and Springer, 1988, J. Cell Biol. 107: 321-331). Thus, ICAM-1 expression appears to be required for the extravasation of immune cells to sites of inflammation. Antibodies to ICAM-1 also block T cell killing, mixed lymphocyte reactions, and T cell-mediated B cell differentiation, indicating that ICAM-1 is required for these cognate cell interactions (Boyd et al., supra). The involvement of ICAM-1 in antigen presentation is shown by the inability of ICAM-1 defective murine B cell mutants to stimulate antigen-dependent T cell proliferation (Dang et al., 1990, J. Immunol. 144: 4082-4091). Conversely, murine L cells require transfection with human ICAM-1 in addition to HLA-DR in order to present antigen to human T cells (Altmann et al., 1989, Nature (London) 338: 512-514). Thus, blocking ICAM-1 function can prevent immune cell recognition and activity during transplant rejection, and can be effective in treating animal models of rheumatoid arthritis, asthma and reperfusion injury.
Expression of ICAM-1 has also been associated with a variety of inflammatory skin disorders such as allergic contact dermatitis, fixed drug eruption, lichen planus, and psoriasis (Ho et al., 1990, J. Am. Acad. Dermatol., 22: 64-68; Griffiths and Nickoloff, 1989, Am. J. Pathology 135: 1045-1053; Lisby et al., 1989, Br. J. Dermatol. 120: 479-484; Shiohara et al., 1989, Arch. Dermatol. 125: 1371-1376). In addition, ICAM-1 expression has been detected in the synovium of patients with rheumatoid arthritis (Hale et al., 1989, Arth. Rheum., 32: 22-30), in the pancreatic B-cells of diabetics (Campbell et al., 1989, P.N.A.S. USA 86: 4282-4286); in thyroid follicular cells of patients with Graves' disease (Weetman et al., 1989, J. Endocrinol. 122: 185-191); in renal and liver allograft rejection (Fault and Russ, 1989, Transplantation 48: 226-230; Adams et al., 1989, Lancet 1122-1125); and in inflammatory bowel disease (IBD) tissue (Springer T, 1990, Nature 346: 425-34).
ICAM-1 expression is also implicated in angiogenesis, which is the formation of new blood vessels from the endothelial cells of preexisting blood vessels. Angiogenesis is a complex process which involves a changing profile of endothelial cell gene expression associated with cell migration, proliferation, and differentiation, which begins with localized breakdown of the basement membrane of the parent vessel. The endothelial cells then migrate away from the parent vessel into the interstitial extracellular matrix (ECM) to form a capillary sprout, which elongates due to continued migration and proliferation of endothelial cells in the ECM. The interactions of the endothelial cells with the ECM during angiogenesis require alterations of cell-matrix contacts which are caused, in part, by an increase in ICAM-1 expression.
Aberrant angiogenesis, or the pathogenic growth of new blood vessels, is implicated in a number of conditions. Among these conditions are diabetic retinopathy, psoriasis, exudative or “wet” age-related macular degeneration (“AMD”), rheumatoid arthritis and other inflammatory diseases, and most cancers. AMD in particular is a clinically important angiogenic disease. This condition is characterized by choroidal neovascularization in one or both eyes in aging individuals, and is the major cause of blindness in industrialized countries.
Several complications commonly seen in type I diabetes also involve expression of ICAM-1. For example, ICAM-1-mediated adhesion of leukocytes to capillary endothelium (also called “leukostasis”) can cause microvascular ischemia in certain tissues of diabetics, such as the retina, peripheral nerves, and kidney. This results in capillary non-perfusion of these tissues, which in turn leads to diabetic retinopathy (Miyamoto K et al. (2000), Am. J. Pathol. 156: 1733-1739; Miyamoto K et al. (1999), P.N.A.S USA 96:10836-1084), neuropathy (Jude E B et al. (1998), Diabetologia 41:330-6) or nephropathy. Miyamoto et al. (1999, P.N.A.S USA 96: 10836-10841) suggest that inhibition of ICAM-1-mediated leukostasis can prevent retinal abnormalities associated with diabetes. However, at least one study reported that the development of diabetic nephropathy in the “Wistar fatty” rat model of diabetes does not appear to involve ICAM-1 expression in glomeruli (Matsui H et al. (1996), Diabetes Res. Clin. Pract. 32:1-9).
ICAM-1 has also been implicated in the onset of macrovascular disease (e.g., coronary artery disease, cerebrovascular disease, and peripheral vascular disease) in type I diabetes, which results in part from accelerated atherosclerosis and increased thrombosis. For example, ICAM-1 has been found in atherosclerotic plaques and is likely involved in the initiation and development of atherosclerosis in diabetics. (Jude E B et al. (2002), Eur. J. Intern. Med. 13:185-189).
ICAM-1 therefore plays an essential role in both normal and pathophysiological processes (Springer et al., 1987, Ann. Rev. Immunol. 5: 223-252). Strategies have therefore been developed to mediate cell adhesion by blocking ICAM-1 function or expression. Such strategies typically employ anti-ICAM-1 antibodies, ligands which competitively block ICAM-1 binding, or antisense nucleic acid molecules directed against ICAM-1 mRNA. However, the agents used in such therapies produce only a stoichiometric reduction in ICAM-1, and are typically overwhelmed by the abnormally high production of ICAM-1 by the diseased or activated cells. The results achieved with these strategies have therefore been unsatisfactory.
RNA interference (hereinafter “RNAi”) is a method of post-transcriptional gene regulation that is conserved throughout many eukaryotic organisms. RNAi is induced by short (i.e., <30 nucleotide) double stranded RNA (“dsRNA”) molecules which are present in the cell (Fire A et al. (1998), Nature 391: 806-811). These short dsRNA molecules, called “short interfering RNA” or “siRNA,” cause the destruction of messenger RNAs (“mRNAs”) which share sequence homology with the siRNA to within one nucleotide resolution (Elbashir S M et al. (2001), Genes Dev, 15: 188-200). It is believed that the siRNA and the targeted mRNA bind to an “RNA-induced silencing complex” or “RISC”, which cleaves the targeted mRNA. The siRNA is apparently recycled much like a multiple-turnover enzyme, with 1 siRNA molecule capable of inducing cleavage of approximately 1000 mRNA molecules. siRNA-mediated RNAi degradation of an mRNA is therefore more effective than currently available technologies for inhibiting expression of a target gene.
Elbashir S M et al. (2001), supra, have shown that synthetic siRNA of 21 and 22 nucleotides in length, and which have short 3′ overhangs, are able to induce RNAi of target mRNA in a Drosophila cell lysate. Cultured mammalian cells also exhibit RNAi degradation with synthetic siRNA (Elbashir S M et al. (2001) Nature, 411: 494-498), and RNAi degradation induced by synthetic siRNA has recently been shown in living mice (McCaffrey A P et al. (2002), Nature, 418: 38-39; Xia H et al. (2002), Nat. Biotech. 20: 1006-1010). The therapeutic potential of siRNA-induced RNAi degradation has been demonstrated in several recent in vitro studies, including the siRNA-directed inhibition of HIV-1 infection (Novina C D et al. (2002), Nat. Med. 8: 681-686) and reduction of neurotoxic polyglutamine disease protein expression (Xia H et al. (2002), supra).
What is needed, therefore, are agents in catalytic or sub-stoichiometric amounts which selectively inhibit expression of ICAM-1, in order to effectively decrease or block ICAM-1-mediated cell adhesion.